Don't unleash
Seriously, war is a much bigger deal in Civ3 than in Civ2. If you want to get involved in a war, make them come to you, weaken them, then attack. Or better yet, build a coalition and stomp on the guy.
My war strategy is to wait, let him crash against my defensive lines, and then hit with a spearhead of knights/cavalry/tanks (whatever is current) backed up by pikemen/riflemen/infantry. Don't try to do too much at once, have a goal in mind before you attack (make it a limited goal: like grab a resource, destroy/take a critical city etc.) It will be virtually impossible to simply walk over a civ anywhere near your technological level.
Cheers,
Dr. Charm